This invention relates to a method of forming a conductive wiring pattern on an insulating substrate by making use of an electrophotography and more particularly to a process of producing printed circuit boards by which the copper-walled holes in the substrate can be formed easily and accurately.
The printed circuit board, which consists of an insulating synthetic resin board with copper conducting paths on both sides, has copper-walled holes that make electrical connections between the two sides and in which terminals or legs of electronic components are mounted and welded by solder.
There are two conventional methods of producing the printed circuit board having copper-walled holes: (A) a tenting method and (B) a hole-plugging method.
The process of the tenting method (A) is shown in FIG. 1. As shown in FIG. 1-a, a hole 4 about 1 mm in diameter is made through the substrate 1 at a desired location. The substrate 1 consists of a synthetic resin board 1a with a thin copper layer 1b, 1b' plated on both sides. The both sides of the substrate 1 and the wall of the hole 4 are plated with a copper layer 3 about 1 .mu.m thick.
Then, as shown in FIG. 1-b, a dry resist film 2 is coated on the surface of the copper layer 3 and a wiring pattern is placed on the dry resist film 2, which is then exposed. The exposed dry resist film 2 is then processed in a solution to produce the wiring pattern of the film 2'. In this treatment a portion 2' of the dry resist film that was exposed to light remains while the unlit portion of the film is removed.
When the substrate is etched, the acid bath erodes the exposed parts of the copper layer 3, 1b, 1b', with the remaining dry resist film 2' on the hole serving as a protective film. And the copper layers covered with the dry resist film 2' at the hole 4 remain as shown in FIG. 1-d.
The final step is to remove the remaining dry resist film 2' from the substrate. With the dry resist film 2' removed, the printed circuit board 1A is obtained in which the plated layer of copper 3 is formed inside the hole 4 as well as the upper and lower surfaces of the substrate around the hole.
The hole-plugging method is performed in the following way. As shown in FIG. 2-a, a hole 4 is made through a substrate 1 which consists of an insulating base plate plated with a copper layer 3 on both sides and on the wall of the hole 4. An acid-resistant ink 5 is charged into the hole 4 and the substrate 1 is polished on the surfaces. A photoresist is applied to a certain portion of the substrate 1 to form a layer of the photoresist 6. A wiring pattern is placed on the photoresist layer 6 and exposed to light. The photoresist layer 6 is washed in the solution to remove unnecessary portions of the photoresist layer, as shown in FIG. 2-b.
The substrate is then etched to remove unnecessary portions of the copper layers 3, 1b, 1b', as shown in FIG. 2-c, with the remaining photoresist layer 6 serving as a protective film. After this, the ink 5 in the hole 4 and the photoresist layer 6 are removed to obtain the printed circuit board 1A with a copper-walled hole 4a, which is the same as the one shown in FIG. 1-e.
In the above process, instead of applying the photoresistant substance 6, the substrate may be printed with the acid-resistant ink by screen printing and bathed in the acid bath to remove the exposed parts of the copper layers. Then, the acid-resistant ink and the ink charged into the hole 4 are removed to obtain the printed circuit board 1A the same as shown in FIG. 1-e.
The above methods of producing the printed circuit board have various drawbacks. In the tenting method, the dry resist film is very expensive, resulting in an increase in the production cost of the printed circuit board. The hole-plugging method requires additional processes of charging and removing ink into/from the hole. The photoresistant substance coated on the substrate has low sensitivity and therefore cannot be processed at a high speed. Moreover, exposing device used in this method is expensive.
In the method using the screen printing, the accuracy of the wiring pattern is low because the screen itself has flexibility.
This invention has been accomplished to overcome the above-mentioned drawbacks encountered with the conventional techniques: i.e., high cost in the tenting method, complicated process in the hole-plugging method, and inaccurate wiring pattern with the screen printing method.